An　extruded　Mg-4Zn-0.2Mn-0.2Ca　alloy　was　developed　as　potential　biodegradable　bone-plate　due　to　its　excellent　biocompatibility.　Long　term　in　vitro　immersion　in　Hank＇s　solution　and　bending　test　were　used　to　evaluate　the　degradability　and　the　mechanical　integrity　of　the　alloy.　The　results　revealed　that　the　degradation　rate　of　the　bone-plate　increased　in　the　first　7　days　and　then　decreased　with　the　prolonged　immersion　time　before　it　finally　reached　a　steady　stage　(about　0.84　mm/a)　after　immersion　for　90　days.　The　bending　strength　after　immersion　for　60　days　was　67.6　MPa,　indicating　that　the　bone-plate　could　support　certain　mechanical　load　after　long　term　degradation.　The　formation　of　corrosion　pits　after　degradation　stemmed　from　the　separation　of　the　continuously　distributed　second　phases　from　Mg　matrix　under　the　action　of　micro-galvanic　couples.　As　a　result,　the　mechanical　performance　of　Mg-4Zn-0.2Mn-0.2Ca　alloy　was　aggravated　owing　to　the　corrosion　holes　on　its　surface.